Hmm, Chemistry...
sorry! I thought you meant muep chem, umep chem or vce chem not enhancement vs chem in general! -sorry!
UMEP Requirements
VCE requirements: Completion of Chemistry 3/4 and Mathematical Methods 3/4 in Year 11, or enrolment in Mathematical Methods 3/4 in Year 12 after completion of Chemistry 3/4 and Mathematical Methods 1/2 in Year 11.
I'm sorry, but I doubt they'll accept you. Mainly because in uni chem you learn why stuff happens not the superficial vce course
MUEP Requirements
Preparatory study: Chemistry
Completion of units 3 and 4 in Year 11 OR Concurrent enrolment in units 3 and 4 in Year 12
This is your only option for unichem of the two extension programs you mentioned, but you have do chem 3/4 next year as well, whereas you only want to do the MUEP chem. (so not possible either)
Now, uni maths:
1. Why drop chem? Do you not like it? Is it that you would prefer maths over chem? What courses are you looking into for uni?
This is because some courses, eg med, have Chemistry as a prerequisite. Dropping chem could mean you miss out on your dream course
2. Hmm, UMEP or MUEP Maths. Both are similar for semester 1 (look at my sig if you're wondering how I know) but MUEP focusses more on series and calculus in semester 2
The thing is, as you've quite rightly pointed out, an extension only counts as a fifth or sixth subject. Also, the marking system for UMEP is different, 90% of the assessment is in the form of the two exams at the end of the year, assignments are worth only 1.11%
At MUEP, assignments are worth more (like 40% iirc) and the exam is... lets just say "not as hard" :P But, it's also a longer trip for you since you said that UMEP is closer to you.
BUT the stinger (sorry) is this:
UMEP MATHS Requirements
VCE requirements: Completion of Mathematical Methods 3/4 in Year 11 and enrolment in Specialist Mathematics 3/4 in Year 12, or concurrent enrolment in Mathematical Methods 3/4 and Specialist Mathematics 3/4 after completion of Mathematical Methods 1/2 and General Mathematics (Specialist Mathematics orientation) 1/2 in Year 11.
I think you can sit a test or something to prove you have all the knowledge of GMS 1/2 but not sure on that one. Sorry can't be more help on this problem
For more info:
Requirements for MUEP: http://www.monash.edu.au/extension/study-options/index.html
Requirements for UMEP: http://futurestudents.unimelb.edu.au/info/school-students/extension-program/subject-areas
Wish you all the best with your studies :)